1. Field of the Invention
The present invention relates to an exposure apparatus for use in the manufacture of devices, such as semiconductor devices, and to a method for manufacturing a device.
2. Description of the Related Art
In general, an exposure apparatus that forms the image of a pattern on a mask or reticle (hereinafter referred to collectively as a “reticle”) on a photosensitive substrate via a projection optical system is used in a lithographic process of the manufacture of semiconductor devices, a liquid crystal display device, a thin-film magnetic, or other devices. With the advance of finer pattern rules and higher packaging densities of an integrated circuit, it is desired for an exposure apparatus that the image of a circuit pattern on a reticle be projected onto the surface of a substrate with higher resolution to expose the surface. The resolution in projecting the image of a circuit pattern depends on the numerical aperture (NA) of a projection optical system and wavelength of exposure light. Typically, a method of increasing the NA of the projection optical system using a constant wavelength of exposure light is performed. The wavelength of exposure light is being reduced more and more. For example, the wavelength of exposure light is shifting from the g-line to the i-line to the oscillation frequency of an excimer laser. For the excimer laser oscillation frequency, the wavelength of exposure light is being reduced from 248 nm to 193 nm, and further to 157 nm. Moreover, the size of an exposure area is increasingly becoming larger.
One example of means for accomplishing these requirements is a stepper that projects light onto a substantially square exposure region of the surface of a substrate so as to form a reduced pattern image thereon and exposes the surface by the step-and-repeat process. Another example of such means is a scanning exposure apparatus that exposes a larger area of a substrate accurately by relatively scanning a reticle and the substrate with high speed using an exposure region that has a rectangular or arc slit shape, a so-called scanner. Such a scanner adjusts a surface shape of the substrate to an optimal exposure image-plane position for each scanning exposure slit, so the scanner has the advantage of reducing the influence resulting from the degree of flatness of the substrate. The scanner also can increase the size of the exposure region and the NA even with a lens equivalent to that used in a stepper. As a result, the scanner is becoming the dominating exposure apparatus. The scanner adjusts the surface of the substrate to an exposure image-plane position for each scanning exposure slit in real time. Thus, before the scanner reaches the exposure slit position, the position of the surface of the substrate is measured with a surface position measuring device and the driving is corrected. The oblique-incidence-light measurement technique is commonly used as the surface position measuring device. In the oblique-incidence-light measurement technique, a beam of light is made to be obliquely incident on the surface of the substrate, and a change in the position of a reflection point of light reflected from the surface of the substrate is measured as the amount of change in the position on a position sensor. The position of the reflection point on the surface of the substrate is referred to as a measurement spot. A plurality of measurement spots are present in the longitudinal direction of the exposure slit, i.e., a direction substantially perpendicular to the scanning direction to measure both the height and the inclination of the surface.
In FIG. 11, three measurement spots are present both in front and behind a scanning exposure slit 900, i.e., measurement spots 901, 902, and 903 and measurement spots 904, 905, and 906 are present in front and behind, respectively. In FIG. 12, five measurement spots are present both in front and behind the scanning exposure slit 900, i.e., measurement spots 907, 908, 909, 910, and 911 and measurement spots 912, 913, 914, 915, and 916 are present in front and behind, respectively. Having a focus measurement system both in front and behind enables scanning for exposure to be performed both from the front and rear directions in FIGS. 11 and 12, thus allowing focus measurement of the substrate to be performed before exposure. A twin-stage exposure apparatus, as illustrated in FIG. 1, has a focus measurement system that has many measurement spots 4a aligned in a line, as illustrated in FIG. 2, aside from an exposure system to find the surface of a substrate 4 in advance. It is necessary to reliably place an entire area to be exposed of the substrate 4 within the allowable depth of focus of a reduction projection optical system, the allowable depth of focus being reduced with an increase in NA. One example method for achieving this aim is disclosed in Japanese Patent Laid-Open No. 9-045608. In this method, the influence caused by local pattern steps (asperities) is eliminated or reduced, and a global shape of the surface is determined. For each measurement spot, the difference between a measurement value (height) and a corresponding value (height) obtained from the global shape the measurement value is determined as a correction value for the measurement value.
With recent expansion of networking technologies, the demand for high performance, such as high integration, reduction in a chip size, speed enhancement, and low power consumption, on a large-scale integration (LSI) is becoming increasingly great. To address the demand, as a result of advancement of finer design rules and multi-layering in conformance with the international technology roadmap for semiconductors (ITRS), a new problem also arises. With finer design rules, the previous process extension techniques could not facilitate speed enhancement of an LSI because they cause an increase in signal delay in an interconnection layer. The delay time in interconnections is proportional to the interconnect resistance and the capacitance between interconnections. Therefore, to advance achieving higher performance, it is necessary to reduce the interconnect resistance and the capacitance between interconnections. Resistance reduction by use of copper interconnections is becoming popular as one way of addressing the interconnect delay time problem. To reduce the capacitance between interconnections, it is necessary to reduce a dielectric constant of an interlayer dielectric. A typical method for reducing the dielectric constant is to introduce holes (the relative dielectric constant is one) to a heat-resistant material. This is called porosification. Copper interconnections are typically formed by a Damascene process because it is difficult to form copper interconnections by dry etching, which has been widely used in other interconnections. In the Damascene process, trenches for interconnections are formed on an interlayer dielectric in advance, and a copper coating is deposited thereon. Then, the copper other than that sunken within the trenches is removed by chemical-mechanical polishing (CMP) to form copper interconnections. In this process, if the mechanical strength of a porous interlayer dielectric is too low, the film is frequently peeled or destroyed inside the film by the stress in CMP. One measure to address this is a method of increasing the mechanical strength by embedding a dummy copper interconnect pattern in a porous interlayer insulating film. The dummy copper interconnect pattern is embedded such that the mechanical strength is uniform over the entire area of the surface of the substrate.
As previously described, the light measurement technique is commonly used as the surface position measuring device of the exposure apparatus. It has been found that the surface position measuring device using the light measurement technique has measuring errors in cases described below. When interference occurs between light reflected from the surface of the resist applied on the substrate and light reflected from the surface of the substrate after having passed through the resist, measuring errors arise. When there is a pattern formed on the surface of the substrate in a front-end process, if light reflected from the surface of the substrate has a distribution according to the distribution of reflectivity of the pattern caused by the influence of the pattern, measuring errors also occur. For either interference or reflectivity, the ratio of the intensity of light reflected from the pattern to the intensity of light reflected from the substrate is increased, thus resulting in a relative increase in the amount of measuring errors. This influence arises when a reflectivity distribution partially varies within a measurement spot region. In addition to a decrease in the depth of focus caused by finer design rules, an increase in the influence of interference caused by a reduction in the film thickness of a resist, and the use of interconnections having high reflectivity, such as copper interconnections, make influence of errors resulting from a reflectivity distribution within the surface of a substrate larger, compared with in the prior art.
An example of the influence of a reflectivity distribution on focus measuring will now be described with reference to FIGS. 5A and 5B and FIGS. 6A and 6B. FIGS. 5A and 5B are diagrams that illustrate the substrate 4 with a resist 503 applied thereon. A pattern 501 is formed in a front-end process and has a high reflectivity. The pattern 501 can be made of, for example, metal. A pattern 502 can be formed from, for example, an interlayer dielectric. The pattern 502 has a reflectivity lower than that in the pattern 501. A beam 504 having a constant beam diameter and uniform intensity distribution within the beam diameter is incident toward the substrate 4. The beam 504 is reflected from the surface of the resist 503, that of the pattern 501, or that of the pattern 502, thus forming reflected beams 505 or 506, each of which exhibits an intensity distribution varying within the beam diameter.
FIGS. 6A and 6B are graphs of intensity distributions in a state where the reflected beams 505 and 506, which are reflected from the surface of each of the resist 503, the pattern 501, and the pattern 502, are focused on a photo detector. In FIGS. 5A and 5B, the beam 504, which has a constant beam diameter and uniform intensity distribution within the beam diameter, is divided into two kinds of components described below. One is components 510 and 511 reflected from the surface of the resist 503. The other is components 512 and 513. The components 512 and 513 have passed through the resist 503, reflected from the surface of the pattern 501 or 502, and gone out of the resist 503 again. That is, each of the reflected beams 505 and 506 includes a combination of the component 510, which is reflected from the surface of the resist 503, and the component 512, which is reflected from the surface of the pattern 501, and a combination of the component 511, which is reflected from the surface of the resist 503, and the component 513, which is reflected from the surface of the pattern 502. Therefore, in FIGS. 5A and 5B, when the ground reflectivity of the pattern 501 is larger than that of the pattern 502, each of the reflected beams 505 and 506 exhibits an intensity distribution 602 varying within a beam diameter, as illustrated in FIGS. 6A and 6B. FIG. 5A corresponds to FIG. 6A, and FIG. 5B corresponds to FIG. 6B. The intensity distribution within the beam diameter of a reflected beam varies according to the position of a pattern arranged within the beam diameter of an incident beam. The surface position measuring device using the light measurement technique is set using a position detecting device (e.g., a charge-coupled device (CCD)) such that the position of a barycenter 601 of a reflected beam that exhibits an intensity distribution varying within the beam diameter is measured as the position of incidence of the reflected beam on the device. At this time, when the substrate lies in the same location, the position of the barycenter of a reflected beam does not vary in normal times. However, because the patterns 501 and 502 are present on the substrate, the barycenter 601 of the intensity distribution of the reflected beam varies depending on a relative positional relationship between the incident beam and the patterns 501 and 502, as illustrated in FIGS. 6A and 6B. Therefore, the measurement values have inherent detection errors according to the arrangement of the patterns 501 and 502. That is, detection errors inherent in a process where pattern structures are different occur. For the same reason, when the pattern 501 is a pattern that allows a beam to pass therethrough, for example, an interlayer dielectric, an interference state with the pattern 502 that does not allow a beam to pass therethrough is changed. Thus, even if they have the same ground reflectivity, the reflectivity is changed by the interference. Also in this case, where such a phenomenon occurs, the barycenter 601 of the intensity distribution of the reflected beam varies, as illustrated in FIGS. 6A and 6B.